The present invention relates to storage apparatus and hierarchical data management thereof.
The cost of storage devices generally corresponds to their speed; faster the device more expensive it is. FIG. 1 provides information about the costs of a plurality of storage devices with respective to their access speed. For example, a semiconductor memory 101 has response speed ranging from nano-seconds to mili-seconds unit, providing extremely fast access speed. On the other hand, in a magnetic tape device 104 has response speed ranging from few seconds to a minute or so, providing relatively low access speed. However, the storage cost per unit of information a magnetic tape is significantly less than (about one several tenth) that of the semiconductor memory. Other storage media, an optical disk storage device 102 and magnetic tape storage device 103, fall somewhere in between the semiconductor device and magnetic tape device in terms of access speed as well as their cost.
Generally, different data have different access frequencies, some being accessed significantly more frequently than other data. Accordingly, the data having a high access frequency are stored in a high speed storage medium, and those having a low access frequency are stored in a low speed storage medium, so that the storage system can be more efficiently utilized.
A hierarchical storage apparatus refers to a storage system having a plurality of storage devices having different access speed, so that the storage apparatus may store frequently accessed information in a high speed storage device and less frequently accessed information in a lower speed storage device. As used herein, the term “storage apparatus” refers to any device having storage capability. Examples of the storage apparatus includes a disk array device including a plurality of disk drives, a storage subsystem including a storage controller and a storage device configured to store data, a storage system including an information processing unit coupled to a storage device via communication link, a disk drive, and a CD-ROM. The terms, “storage subsystem” and “storage system” are used interchangeably herein unless specifically excluded from the context. For example, in certain instances, the storage system refers to a system including a storage subsystem, e.g., the storage subsystem is a disk array device and the storage system is a network system including the disk array device.
Various methods have been proposed for transferring data between respective storage media within a hierarchical storage apparatus. One method involves moving data stored in a high speed device to a lower speed device if a predetermined time has elapsed without the data being accessed. In another method, higher priority data are allocated to a higher speed storage medium and lower priority data are allocated to a lower speed storage medium.